A frequency converter is typically a device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency. However, frequency converters inevitably experience malfunctions during use. When analyzing these malfunctions, understanding the components of the frequency converter's main circuit can be very helpful in quickly and effectively resolving them. Therefore, we will now analyze the components of the frequency converter's main circuit.
I. Cross-sectional to direct transformation section
1. VD1 to VD6 form a three-phase rectifier bridge to convert AC to DC. If the three-phase line voltage is UL, then the rectified DC voltage UD is: UD = 1.35UL
2. Function of filter capacitor CF:
(1) Filter out the voltage ripple after full-wave rectification;
(2) Keep the DC voltage balanced when the load changes.
Because of capacitance and voltage withstand capability, filter circuits typically consist of several capacitors connected in parallel to form a group, or two capacitor groups connected in series. Since the characteristics of the two groups of capacitors cannot be exactly the same, a voltage divider resistor RC1 and RC2 with equal resistance is connected in parallel to each capacitor group.
3. Current-limiting resistor RL and switch SL
Function of RL: The inrush current is relatively large when the inverter is first closed. Its function is to control the inrush current and keep the charging current of capacitor CF within a certain range for a period of time after the switch is closed.
SL function: When CF is charged to a certain voltage, SL closes, short-circuiting RL. Some frequency converters use thyristors instead (as shown by the dashed line).
4. Power indicator HL
Function: In addition to serving as a power indicator for the frequency converter, it also serves as an indicator of whether the frequency converter has power after it is powered off (operations such as disconnecting wires can only be performed after the light goes out).
II. Energy Consumption Circuit Section
1. Braking resistor RB
As the frequency of a frequency inverter decreases, it enters a regenerative braking state, and the regenerative energy is stored in capacitor CF, causing the DC voltage to rise continuously, even reaching a dangerous level. The function of resistor RB is to dissipate this regenerative energy. Some frequency inverters have external resistors with external terminals (such as DB+, DB-).
2. Braking unit VB: Composed of a GTR or IGBT and its driving circuit. Its function is to provide a path for the discharge current IB to flow through RB.
III. Orthogonal Transformation Section
1. Inverter diodes V1 to V6: These form an inverter bridge, converting the DC power rectified by VD1 to VD6 into AC power. This is the core component of the frequency converter.
2. Freewheeling diodes VD7～VD12
effect:
(1) The motor is an inductive load, and its current contains a reactive component, providing a "channel" for the reactive current to return to the DC power supply;
(2) When the frequency decreases and the motor is in regenerative braking state, the regenerative current is rectified by VD7～VD12 and returned to the DC circuit;
(3) During the V1 to V6 inverter process, the two inverter tubes on the same bridge arm are constantly in the on and off states. During this commutation process, VD7 to VD12 are also required to provide a path.
IV. Buffer Circuit
During the instantaneous switching on and off of the inverter diodes, the rate of change in voltage and current is relatively large, which could potentially damage all inverter diodes. Therefore, a buffer circuit is connected next to each inverter diode to slow down the rate of change in voltage and current.
1. C01～C06
At the instant each inverter transistor V1-V6 transitions from conduction to cutoff, the voltage between its collector (C) and emitter (E) rapidly rises from 0V to the DC voltage UD. An excessively high voltage rise rate will damage the inverter transistors. The function of capacitors C01-C06 is to reduce this excessively high voltage rise rate during the turn-on to cutoff process, thus preventing inverter damage.
2. R01～R06
At the instant inverter diodes V1 to V6 turn from on to off, the voltage charged by capacitors C01 to C06 (equal to UD) discharges V1 to V6. The initial value of this discharge current is very large and is superimposed on the load current, potentially damaging the inverter diodes. The function of resistors R01 to R06 is to control the discharge current of C01 to C06 at the instant the inverter diodes turn on.
3. VD01～VD06
The connection of R01 to R06 will affect the effect of C01 to C06 in reducing the voltage growth rate when V1 to V6 are turned off. After VD01 to VD06 are connected, R01 to R06 will not function during the V1 to V6 turn-off process; while during the V1 to V6 turn-on process, the discharge current of C01 to C06 will be forced to flow through R01 to R06.
Through the above content, we have gained a general understanding of the components of the inverter's main circuit. Hopefully, this understanding of the inverter's main circuit structure will help in resolving inverter malfunctions and provide everyone with a new perspective on inverters.
